A typical use of such loose-material grates is for the cooling of fired cement clinkers by means of air; the invention being illustrated using this as an example. However, the invention can be applied very generally to each grate arrangement in which loose material received on the grate surface is treated by means of a gas flowing from below through the grate surface.
In the case of loose-material grates, one basically differentiates between a so-called chamber ventilation arrangement by means of ventilation chambers arranged below the grate surface, and a so-called channel ventilation arrangement (also called the direct ventilation), in which the individual rows of grates are designed as individual air beams and are connected to a separate air-connection channel (see, for example, DE 33 32 592). The channel ventilation arrangement is mainly utilized in the field of stepped thrust grates in which fixed rows of grates alternate with rows of grates movable in a transporting direction of the loose material.
The channel ventilation arrangement offers, compared with the chamber ventilation arrangement, the possibility of carrying out, at least in the area of the individual rows of grates, a ventilation control by means of flaps or the like arranged in the associated air-supply channel. The channel ventilation arrangement adapts the grate better to varying ventilation demands in different grate areas, for example, to a cooling-performance demand decreasing in a transporting direction of the grate. However, it has been found that such a controllable channel ventilation does not permit a sufficiently quick reaction to changed conditions in the loose-material bed. For example, in an air breakthrough or the like, the associated control members always react with an unacceptable time delay to an already occurred change, for example an air breakthrough.
Already during the introduction of the air-beam technique with the possibility of the channel ventilation arrangement, the knowledge was therefore mainly taken into consideration that the grate resistance is the deciding operating factor of the grates in order to achieve a constant bed ventilation and to, for example, prevent damaging air breakthroughs in the loose-material bed. The air-beam technique makes it possible to make the grate resistance high and to correspondingly increase the pressure of the cooling air in the air beam without having to at the same time accept thereby high air losses in the damaging gaps existing between the adjacent rows of grates or between these and the lateral grate boundaries.
When the resistance coefficient of the grate is high and the resistance portion of the grate relative to the entire resistance comprising the bed resistance is large, then changes in the bed resistance, as they occur, for example, suddenly in the case of air breakthroughs, have a lesser effect than in the case of a low grate resistance, as can be proven. The resistance increases in the case of an air breakthrough at the breakthrough point proportionally equally in time and thus essentially blocks the air so that the air breakthrough does not increase further and balancing flows from other grate areas are essentially avoided.
When the air-beam technique was introduced, the necessity of blocking air for the above-mentioned undesired gaps, in particular the thrust gaps between fixed and movable rows of grates, became very quickly obvious. This blocking air blocks the gaps against falling material, which causes, for example in a red-hot state, damage in the lower grate or which is absorbed again by the cooling air as dust and causes wear in the grate. The blocking air is generally supplied from below to the grate through additional ventilation chambers provided solely for this purpose or as a branch of the cooling air provided for the air beam. Due to an increasing wear and thus larger gaps with time, more and more blocking air was needed so that in older systems the unintended blocking-air portion was above the air portion desired for the channel ventilation through the air beams.
With this the principle of the channel ventilation led to ad absurdum.
Besides the blocking-air problems, a further deficiency of the air-beam technique is that a sufficiently small-cell division of the grate surface into separately defined areas cannot be achieved. A small-cell division of the grate surface would be desirous in order to, on the one hand, prevent balancing flows over larger areas and in order to, on the other hand, enable a freer design of the blow-off profile over the grate surface and thus an optimum adaptation to varying cooling needs. The technical expense for the air-supply channels and control valves associated with the individual cells stands in the way of such a small-cell division. Aside of the construction expense there also is the considerable amount of expense associated with adjustment. Common controls function in such a manner that first an interference value must occur before the adjustment can occur. A premature recognition of the interference values within small cells through a monitoring of the amounts of air would be too complicated and expensive.
The blocking-air problems have been essentially overcome by a special, wear-resistant suspension of the movable structure of the grate carrying the moved rows of grates (DE 38 44 493) and an improved fastening of the grate elements (DE 44 41 009). The possibility to realize an optimized grate resistance, separately for the conditions of individual grate sections, for example with a right/left differentiation adapted to different grain sizes or with a dropping rate of air flow in direction of the loose-material conveyance, fails, however, because of the already above-mentioned technical expense for a small-cell division of the grate surface.